The present invention relates to an electrophotographic printer utilizing a means for forming an electrostatic latent image corresponding to information to be printed, on an image-bearing member, a means for developing the electrostatic latent image with a magnetic developer containing magnetic toner, a means for transferring the developed image to a recording medium, and a means for heat-fixing the developed image to the recording medium, and more particularly to an electrophotographic printer which has reduced thinness and improved portability.
In a conventional electrophotographic printer, an electrostatic latent image corresponding to a printer output (information to be printed) is generally formed on a cylindrical photosensitive drum, brought into contact with a magnetic brush of a magnetic developer conveyed on a developing roll containing a permanent magnet roll and disposed opposite the photosensitive drum, so that it is developed (visualized as a toner image). The developed image is then transferred onto a recording paper and heat-fixed.
The magnetic developer usable for a magnetic brush method, as mentioned above, is in many cases a two-component developer consisting of a magnetic carrier and a non-magnetic toner. However, in the case of using such a two-component developer, a concentration sensor and other members are required, making it difficult to miniaturize an electrophotographic printer satisfactorily. Accordingly, a one-component developer consisting of a magnetic toner, or a magnetic developer consisting of a magnetic toner and a magnetic carrier is mostly used for the electrophotographic printer.
With the above magnetic developer, the electrophotographic printer can be miniaturized to some extent, but there is a limit in the conventional electrophotographic printer. That is, since the conventional electrophotographic printer has a heat-fixing means comprising a heat roll containing a heat source and a pressure roll for pressing the recording paper to the heat roll, and since these rolls are provided with paper-separating fingers and other accessories, it is impossible to reduce the height of the roll pair of the heat-fixing means drastically. Accordingly, it has been impossible to make the conventional electrophotographic printer have an extremely reduced thickness.
A charging means and a transfer means in a conventional electrophotographic printer are usually of corona charging type which utilizes corona discharge occurred by applying high voltage (DC 5-8 kV) to a metal wire. In these means, however, ozone and nitrogen oxides are generated by the corona discharge while being accompanied with unpleasant odor which leads to environmental pollution. Also, the ozone and nitrogen oxides generated by the corona discharge gradually deteriorate the surface of the photosensitive drum, resulting in poor image resolution. In addition, dust attached to the wire in the charging means affects the image quality, causing white spots and black streaks on the resulting image.
In a transfer means utilizing corona discharge, a toner image on the photosensitive drum is electrostatically transferred onto a recording medium by applying a charge to the rear of the recording medium with a polarity opposite to that of the toner image This means that the transferability varies with the electric resistance of the recording medium, and further with the humidity that affects the electric resistance of the recording medium. Specifically, the transferability of the image is low when the electric resistance of a recording medium is low.
Further, in the charging means and transfer means which utilize the corona discharge, the electric current actually supplied to the photosensitive drum or recording medium is 5-30% of that supplied from the power source, because the rest streams into a shield plate. Due to this low electric efficiency, it is inevitable to consume considerably greater amounts of electricity than that actually needed in the charging means or transfer means, thereby requiring a set-up transformer of greater capacity.
In the meantime, demands for a portable (handcarried) electrophotographic printer as well as for a stationary one have greatly increased recently. Although such a portable electrophotographic printer has already been commercialized for a type using heat-sensitive papers, the information or image recorded on the heat-sensitive papers will disappear as time passes. Accordingly, for the purpose of printing information which should be kept for a long period of time, the heat-sensitive paper type electrophotographic printer is not suitable.